From: Charles Mills <cmills@...>
Date: 2004-08-04T09:47:16+09:00
Subject: Re: Fastest way to reverse-process a string?


On Aug 3, 2004, at 5:41 PM, Lloyd Zusman wrote:

> Joel VanderWerf <vjoel@PATH.Berkeley.EDU> writes:
>
>> Lloyd Zusman wrote:
>>> In a ruby program I'm writing, I want to parse a string in reverse 
>>> and
>>> feed each of its individual characters through a state machine.
>>> A "hammer and tongs" solution looks like this:
>>>   string.reverse.split(//).each {
>>>     |ch|
>>>     feedIntoStateMachine(ch)
>>>   }
>>> However, this seems rather inefficient.  Does anyone know of a faster
>>> algorithm for this in ruby?
>>
>> String#each_byte would avoid the creation of the array.
>
> Thank you very much.
>
> Does anyone know of a way to avoid 'reverse', as well?  I discovered a
> 'reverse_each' method for arrays, but there doesn't seem to be a
> corresponding String#reverse_each_byte.  Without that, I would have to
> incur the expense of reversing the string before applying each_byte to
> it.
>
> Any ideas?
>
You could write one in C... this should work, slight modification of 
each_byte code... untested:
static VALUE
rb_str_reverse_each_byte(str)
     VALUE str;
{
	long i = RSTRING(str)->len;

	while (i > 0) {
		i--;
		rb_yield(INT2FIX(RSTRING(str)->ptr[i] & 0xff));
	}
	return str;
}

> -- 
>  Lloyd Zusman
>  ljz@asfast.com
>  God bless you.
>
>